The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Known combustion by-products in an exhaust gas feedstream include carbon monoxide (CO), oxides of nitrogen (NOx), and particulate matter (PM), among others. Unburned hydrocarbons (HC) are also present in engine-out emissions. Operating the engine at varying air/fuel ratios including rich, lean and stoichiometric ratios produce different proportions of the by-products and HC, e.g., increased NOx emissions during lean engine operation.
Lean of stoichiometry engine operation can improve fuel efficiency, but may require additional catalytic material to reduce or otherwise treat NOx emissions. Known aftertreatment systems for internal combustion engines operating lean of stoichiometry can include a three-way catalytic converter followed by a lean-NOx reduction catalyst, also referred to as a lean NOx trap (LNT device), which can be used in concert with other exhaust aftertreatment devices, e.g., a selective catalytic reduction catalytic device (SCR device). Known three-way catalytic converters (TWC) function to reduce engine-out HC, CO, and NOx emissions during stoichiometeric engine operation. However, aftertreatment devices including the LNT device and the SCR device can require reductants to reduce NOx molecules within the exhaust gas feedstream during lean engine operation.
A known method for supplying reductant to the aftertreatment system includes injecting reductants into the aftertreatment system, e.g., a hydrocarbon selective catalytic reduction system (HC—SCR). Hydrocarbon reductants typically require a 10:1 reductant to NOx ratio in the exhaust gas feedstream for NOx reduction. Oxygenated hydrocarbon reductants, e.g., ethyl alcohol-based, however, require a 5:1 reductant to NOx ratio for NOx reduction. Ammonia based reductant systems can require a reductant storage tank on the vehicle with periodic refilling and may be prone to freezing in cold climates requiring additional heaters and insulation. Another known method to supply reductant includes configuring the TWC to produce ammonia during engine operation. This method requires a control scheme to alter an air/fuel ratio for combustion in the engine thereby reducing fuel efficiency.